When charging a rechargeable battery, for example a NiCd battery, an electrical voltage greater than the open circuit output voltage of the battery is applied to the terminals of the battery so that a current flow through the battery is generated. The current causes a chemical process to take place in the battery by which energy is stored in the battery.
When a battery is fully charged, the chemical process stops and energy supplied to the battery thereafter will be converted into heat. Since rechargeable batteries are housed. in sealed containers, heat generation leads to increased pressure in the battery causing chemical destruction of the battery whereby the energy storing capacity of the battery is reduced.
Thus, when charging a battery, it is important to fully charge the battery and to stop charging the battery before generation of heat in the battery severely reduces the energy storing capacity of the battery.
Typically, the voltage across the battery terminals increases linearly during charging. However, when the battery is close to being fully charged, the voltage increases more steeply to a maximum, indicating that the state of being fully charged has been reached. Continued charging leads to a voltage drop because of the temperature increase as the temperature coefficient of the voltage is negative. Correspondingly, the charging current, typically, falls to a minimum at full charge and then increases.
In WO 92/15142 a method is disclosed in which charging of a battery can be precisely terminated, i.e. after having fully charged the battery and before reducing the capacity of the battery. The method comprises determining a remaining charging time during charging based on determined charging parameter values and desired charging parameter values.
Charging parameters may include the voltage across the battery, the charging current supplied to the battery, and/or the temperature of the battery. Empirically, it has been shown that the shape of curves of these parameters as a function of time correlates with the distance to the point in time where it is desirable to terminate charging. Having determined the shapes of the curves, the time needed to fully charge the battery may be determined from comparison with respective reference curves.
In U.S. Pat. No. 5.541.490 a battery pack for a portable computer is disclosed, comprising a controller for controlling output current of a battery charger. The control signal generated by the controller is a fixed frequency pulse width modulated signal, the duty cycle of which being determined in response to the voltage and temperature of the battery being charged and the charging current being supplied to the battery. The control signal is low pass filtered at the charger for provision of a DC value of the signal that is proportional to the duty cycle of the signal. The DC value is entered into an input of a summing amplifier, the output of which provides a feedback control signal to the battery charger.